1. Field of the Invention
The present invention is directed toward the field of manufacturing integrated circuits. The invention is more particularly directed toward an improved method and apparatus for introducing process and purge material in a deposition process system.
2. Description of the Related Art
Presently, aluminum is widely employed in integrated circuits as an interconnect, such as plugs and vias. However, higher device densities, faster operating frequencies, and larger die sizes have created a need for a metal with lower resistivity than aluminum to be used in interconnect structures. The lower resistivity of copper makes it an attractive candidate for replacing aluminum.
There are a few well established techniques for depositing copper including, chemical vapor deposition ("CVD"), physical vapor deposition ("PVD") and electrodeposition. In one known method, chemical vapor deposition of copper is achieved by using a precursor material known as Cupraselect.RTM., which has the formula Cu(hfac)L. Cupraselect.RTM. is a registered trademark of Schumacher of Carlsbad, Calif. The Cupraselect.RTM. consists of copper (Cu) bonded to a deposition controlling compound such as (hfac) and a thermal stabilizing compound (L). The (hfac) represents hexafluoroacetylacetonato, and (L) represents a ligand base compound, such as trimethylvinylsilane ("TMVS"). This precursor material is vaporized and flowed into a deposition chamber containing a wafer. In the chamber, the vaporized precursor material is heated at the wafer's surface. At the desired temperature the following reaction results:
2 Cu(hfac)L.fwdarw.Cu+Cu(hfac)2+2L (Eqn. 1)
The resulting copper (Cu) deposits on the surface of the wafer. The byproducts of the reaction (i.e., Cu(hfac).sub.2 and (2L) are purged from the chamber which is maintained at a vacuum during wafer processing.
In some instances, a Cupraselect.RTM. blend is used as the precursor material. The Cupraselect.RTM. blend is Cu(hfac)L with extra (hfac) blended with the Cu(hfac)L for additional stability. One problem associated with using blended Cupraselect.RTM. for CVD is the delivery of the material from its liquid storage ampoule to the process chamber in which the CVD occurs. Typically, the liquid Cupraselect.RTM. (or blend) is stored in bulk and is passed to the process chamber via a direct liquid injection (DLI) system. The DLI system vaporizes the precursor material in close proximity to the wafer. Such a system is seen and described in commonly assigned patent application entitled, "Method and Apparatus for Improved Control of Process and Purge Material in a Substrate Processing System" by Schmitt, et al. filed Jul. 21, 1998. After vaporization, the Cupraselect.RTM. is pumped into the process chamber via a carrier gas such as Argon, Helium or other inert gases. This pumping action tends to pull a high concentration of TMVS out of the Cupraselect.RTM. blend leaving the less stable copper and hfac. Under these conditions, deposition is likely to occur at undesirable locations. For example, deposition can occur near the vaporizer, valves, process chamber showerhead, and the like. Deposition changes the dimensions of these critical system components which degrades performance of the chamber and the resultant deposition layer on the wafer. Additionally, unwanted deposition may flake off during the deposition process which can render a processed wafer faulty or unusable. A maintenance cycle would then have to be run on the process chamber to replace or clean the chamber which reduces system productivity.
Similar difficulties exist at other times within the CVD system. For example, when a precursor material ampoule is nearly empty and is to be replaced, the transfer lines between the ampoule and the process chamber must be pumped out. Similar to the process pumping, transfer line purge pumping pulls greater concentrations of TMVS out of the residual Cupraselect.RTM. blend remaining in the transfer lines leaving the less stable Cu and (hfac) which can cause particles (deposition) to form in the transfer lines or in the valve that accepts the ampoule. When a new ampoule (under pressure) is installed, the flow of high pressure liquid Cupraselect.RTM. can easily dislodge particles formed in the lines and valves and carry such particles down to other delivery equipment or the process chamber.
Additionally, during the deposition process, edge purge gas is provided at the wafer to keep copper from depositing on the edge (exclusion zone) and backside of the wafer. The edge purge gas (typically an inert gas such as Argon) diffuses around the edge of the wafer to redirect the flow of any process gases such as vaporized Cupraselect.RTM. away from edge of the wafer. Typically, the wafer is a silicon or silicon dioxide based substrate. In such substrates, copper can easily diffuse into the wafer thereby introducing additional conductive particles. Such contamination can short devices (i.e., gate structures) being fabricated on the wafer. The physical interaction of the purge gas on the process gas that reduces deposition at the exclusion zone and wafer backside. Additionally, metal etching of copper is not easily accomplished; other processes such as Chemical Mechanical Polishing (CMP) are used instead. Unfortunately, CMP can also create particles at the edge of the wafer that can be transferred to subsequent chambers.
Accordingly, it is desirable to provide an apparatus and method for improved control and handling of precursor material and purge additives in a substrate process system to reduce the likelihood of deposition or particle formation within the system as well as enhance the edge purge gas capabilities.